Polyaniline (PANI), when doped with small-molecule acids, is an attractive
candidate for organic and polymer electronics because of its high electrical conductivity.
Its utility as functional components in electrical devices, however, has been severely
restricted because such PANI has limited processibility stemming from its limited
solubility in common solvents. To overcome this barrier, we have developed water
dispersible PANI that is template polymerized in the presence of a polymer acid, poly(2-acrylamido-2-methyl-1-propanesulfonic acid), or PAAMPSA. The polymer acid serves
two roles: it acts as a dopant to render PANI conductive and excess water soluble pendant
groups provide dispersibility of PANI in aqueous media. While the introduction of
polymer acids renders the conducting polymer processible, such gain in processibility is
often accompanied by a significant reduction in conductivity. As such, PANI that is
doped with polymer acids has only seen limited utility in organic electronics. Given the promise of conducting polymers in organic electronics in general, this thesis focuses on
the elucidation of processing-structure-property relationships of PANI-PAAMPSA with
the aim of ultimately improving the electrical conductivity of polymer acid-doped PANI.
By controlling the molecular weight and molecular weight distribution of the
polymer acid template, we have improved the conductivity of PANI-PAAMPSA from 0.4
to 2.5 S/cm. The conductivity increases with decreasing molecular weight of
PAAMPSA, and it further increases with narrowing the molecular weight distribution of
PAAMPSA. Strong correlations between the structure and the conductivity of PANI-PAAMPSA
are observed. In particular, the crystallinity of PANI increases with
increasing the conductivity of PANI-PAAMPSA. Given that the crystallinity qualifies
the molecular order in PANI-PAAMPSA, we observe a linear correlation between
molecular order and macroscopic charge transport in PANI-PAAMPSA.
PANI-PAAMPSA forms electrostatically stabilized sub-micron particles during
polymerization due to strong ionic interactions between the sulfonic acid groups of
PAAMPSA and aniline. When cast as films, the connectivity of these particles must
play an important role in macroscopic conduction. The size and size distribution of
PANI-PAAMPSA particles is strongly influenced by the molecular characteristics of
polymer acid template. Templating the synthesis of PANI-PAAMPSA with a higher
molecular weight PAAMPSA results in larger particles, and templating with a
PAAMPSA having a larger molecular weight distribution results in a large size
distribution in the particles. Because conduction in PANI-PAAMPSA films is governed
by how these particles pack, the macroscopic conductivity of PANI-PAAMPSA films
increases with increasing particle density, that is reducible from the molecular characteristics of PAAMPSA. Moreover, PANI-PAAMPSA particles are structurally
and chemically inhomogeneous. The conductive portions of the polymer preferentially
segregate to the particle surface. Conduction in these materials is therefore mediated by
the particle surface and conductivity thus scales superlinearly with particle surface area
per unit film volume.
We further have improved the electrical conductivity of PANI-PAAMPSA by
more than two orders of magnitude via post-processing solvent annealing with
dichloroacetic acid (DCA). Since DCA is a good plasticizer for PAAMPSA and its pKa
is lower than that of PAAMPSA (pKas of DCA and PAAMPSA are 1.21 and 2.41,
respectively, at room temperature), DCA can effectively moderate the ionic interactions
between PANI and PAAMPSA, thereby relaxing the sub-micron particulate structure
arrested during polymerization. PANI-PAAMPSA can thus rearrange from a “compact
coil” to an “extended chain” conformation upon exposure to DCA. Efficient charge
transport is thus enabled through such “extended chain” PANI-PAAMPSA structure.
DCA-treated PANI-PAAMPSA exhibits an average conductivity of 48 S/cm. The DCA
treatment is not only specific to PANI-PAAMPSA. This treatment can also enhance the
conductivity of commercially-available poly(ethylene dioxythiophene) that is doped with
poly(styrene sulfonic acid), or PEDOT-PSS. Specifically, DCA-treated PEDOT-PSS
exhibits a conductivity of 600 S/cm; this conductivity is the highest among polymer acid-doped
conducting polymers reported so far.
PANI-PAAMPSA can effectively function as anodes in organic solar cells
(OSCs) whose active layer is a blend of poly(3-hexylthiophene), P3HT, and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Specifically, the OSCs with PANI-PAAMPSA anodes exhibit an average short circuit current density of 1.95 mA/cm², open circuit
voltage of 0.52 V, fill factor of 0.38, and efficiency of 0.39 %. The use of DCA-treated
PANI-PAAMPSA as anodes increases device performance (i.e., short circuit current
density and thereby efficiency) of OSCs by approximately two and a half fold. The
OSCs with DCA-treated PANI-PAAMPSA anodes exhibit short circuit current density
and efficiency as high as 4.95 mA/cm² and 0.97 %, respectively.
We demonstrated several factors that govern the electrical conductivity of
polymer acid-doped conducting polymers. Design rules, such as those illustrated in this
study, can enable the development of conducting polymers that is not only easily
processible from aqueous dispersions, but also sufficiently conductive for electronic
applications, and should bring us closer to the realization of low-cost organic and polymeric electronics.